1. Field of the Invention
The present invention relates to a system for coding a digital video signal, and, more particularly, to a forced intra-frame coding method for coding to rapidly and precisely reproduce forced intra-frame coded video data.
2. Discussion of Related Art
Generally speaking, apparatuses for digital processing of video and audio signals, such as a high-definition television sets, high-definition video cassette recorders, digital video cassette recorders, and digital camcorders, contain a system for coding, transmitting and storing the video and audio signals as digital data and, thereafter, decoding and reproducing the coded data. Such coding/decoding systems are increasingly covered by international standardization agreements, which agreements are being implemented at a brisk pace so that standard formats appropriate for individual application fields are available.
FIG. 1A is a block diagram of a general coding system, whose operation employs the well-known intra-frame and inter-frame coding method. A brief discussion is provided below.
For intra-frame coding, switches 22 and 23 are both turned off so that a subtractor 11 sends externally input video data to an orthogonal transformer 12. In other words, the external video data is fed to a variable-length encoder 14 via orthogonal transformer 12 and a quantizer 13. Thus, in intra-frame coding, a buffer 15 transmits the video data, which is externally input to subtractor 11 and then pulse-code modulated, to a receiver.
Meanwhile, for inter-frame coding, switches 22 and 23 both are turned on so that subtractor 11 subtracts video data supplied from a motion compensator 21 from the externally input video data. As a result, a difference signal of the input video data and video data previous to the input video data by one frame is applied to orthogonal transformer 12. An adder 18 adds the data applied via an inverse quantizer 16 and an inverse orthogonal transformer 17 and the output data of motion compensator 21, and outputs the added result to a frame memory 19. In inter-frame coding, the difference data obtained from subtractor 11 is coded, stored in buffer 15, and then output to the receiver. Such PCM or differential PCM techniques are well-known in the art.
FIG. 1B is a block diagram of a general decoding system, which decodes and reproduces the video data coded by the exemplary coding system of FIG. 1A. The general decoding system is composed of a variable-length decoder 31, an inverse quantizer 32, an inverse orthogonal transformer 33, an adder 34, a motion compensator 35, and a frame memory 36 so as to decode the video data coded by the FIG. 1A apparatus. In the case of intra-frame coded data, the output data of inverse orthogonal transformer 33 passes through adder 34 as a switch 37 is turned off. In the case of inter-frame coded data, adder 34 adds the output data of inverse orthogonal transformer 33 and data applied from motion compensator 25 provided via turned-on switch 37. The decoding system of FIG. 1B is commonly used. Thus, further detailed discussions of the operation will not be provided.
FIG. 2A illustrates one frame of video data coded by FIG. 1A apparatus. As shown in FIG. 2A, one frame, made up of X(column).times.Y(row) pixels, is divided into P(column).times.Q(row) macro blocks (hereinafter referred to as MB) which are predetermined units for use in coding. The MB consists of m(row)-by-n(column) blocks, each having N-by-N pixels. Each of the MBs has luminance data blocks Y and color data blocks C. If one MB is made up of four luminance data blocks Y and two color data blocks C, N.times.N pixel blocks of the MB are coded in the order of the arrow shown in FIG. 2B.
In such coding systems, when power is turned on, a channel is changed, or an error is produced in the middle of coding, the original image cannot be restored in an inter-frame coding, i.e., wherein a frame is reconstructed by adding a decoded difference signal and a previous frame signal. In order to enable the decoder to normally restore an image, the coder performs intra-frame coding with respect to part of the data of the overall frames. Units for intra-frame coding are the macro blocks. MBs which are forcibly intra-frame coded by a predetermined period are called `forced intra MBs`.
FIGS. 3A, 3B and 3C illustrate forcibly intra-frame coded frames of video data. Forcibly intra-frame coded MBs are represented by the hatched columns in FIGS. 3A, 3B and 3C. First, in FIG. 3A, the (i)-th frame of video data is coded sequentially from left to right. For coding of respective frames, macro blocks of the uppermost part of a frame are coded from left to right. As shown in FIGS. 3A, 3B and 3C, forced intra-frame coding is performed for every predetermined period of MB. The remaining macro blocks are inter-frame coded. When one row of coding is finished, a next row of coding is carried out in the same way. Likewise, respective macro blocks are coded.
When the (i)-th frame of coding is finished, forcibly intra-frame coded MB columns of the (i+1)-th frame, i.e., the next frame, are shifted by one column to the right, as shown in FIG. 3B. Likewise, when the (i+1)-th frame of coding is finished, the forcibly intra-frame coded MB columns of the (i+2)-th frame, i.e., the next frame, are shifted by one column, as shown in FIG. 3C.
The data coded in the above-described way is restored by the FIG. 1B apparatus. The inter-frame coded macro blocks are restored on the basis of a normally renewed previous frame of video data. If the previous frame of video data is not in a state of renewal, the inter-frame coded macro blocks are not normally restored. Therefore, if there is horizontal movement in the blocks, a time delay is required for normal restoration of the inter-frame coded macro blocks since the previous frame of video data cannot be normally renewed. That is, time delay is required until the previous frame of video data, for use in the restoration of the inter-frame coded macro blocks, is completely renewed. Since there is more horizontal movement than vertical movement in most images, the time delay becomes more serious.
A technique for enabling high-speed playback of encoded video information is disclosed in EP No. 0,536,630 A2, which was laid open on Apr. 14, 1993, to Niimura. According to this reference, one refresh block is transmitted for each horizontally disposed 11 super blocks in a data transmission sequence different from a sequence of reading pixels arranged in accordance with scanning lines. Then, in a high-speed playback mode, the refresh block data is serially written in PG,6 a frame memory, and the data stored in the frame memory is read out in the transmission sequence of the pixels in accordance with the scanning lines. This allows a good-quality image to be easily obtained during high-speed playback.